Energy breakthrough making it cheaper - everything needs energy to function, that's probably an obvious pick.
Internet in rural places - access to internet is needed for everything: new jobs, entertainment, services, goods etc. If there'll be a way to access internet in rural places all over the world - it'll probably be one of the best companies (thinking starlink or something similar, since preferably you'll need low latency).
Content production or content delivery - people will always want entertainment, ie games, movies or streams. It is very far from ideal both in production and delivery. Solving it will give insane reach and insane amounts of money.
Reimagination of hiring and changing professions.
Probably another social network - people are not satisfied, someone has to try.